Apparatus for controlling back pressure in an extruder

ABSTRACT

A pressure control valve or apparatus adapted for controlling the pressure of a viscous fluid in a conduit by producing a pressure drop which is substantially directly proportional to the valve stem position. The valve or apparatus comprises in combination a valve body having a streamlined flow channel which has a narrow section of substantially uniform cross-sectional dimension opening into a wide section of substantially larger cross-sectional dimension and a plunger which is slidably mounted within the channel. The plunger is adapted for movement within the wide section and narrow section of the flow channel wherein the clearance distance between the sidewall of the narrow section and the plunger surface extending into the narrow section is substantially constant. When the plunger is moved reciprocally into the narrow section the changes in pressure drop across the valve or apparatus are substantially directly proportional to the axial extent to which the plunger lies within the narrow section.

United States Patent Skibo et al.

[1 1 3,647,344 51 Mar.7, 1972 APPARATUS FOR CONTROLLING BACK PRESSURE IN AN EXTRUDER Andrew D. Skibo, Monson; Chester L.

Woodworth, East Longmeadow, both of Mass.

Inventors:

Monsanto Company, St. Louis, Mo.

Mar. 16, 1970 Assignee:

Filed:

Appl. No.:

References Cited UNITED STATES PATENTS 3,283,041 11/1966 Sommerfeld ..l8/l2STX Primary Examiner-Henry T. Klinksiek Attorney-William J. Farrington, James C. Logomasini, Richard W. Stemberg and Neal E. Willis [5 7] ABSTRACT A pressure control valve or apparatus adapted for controlling the pressure of a viscous fluid in a conduit by producing a pressure drop which is substantially directly proportional to the valve stem position. The valve or apparatus comprises in combination a valve body having a streamlined flow channel which has a narrow section of substantially uniform cross-sectional dimension opening into a wide section of substantially larger cross-sectional dimension and a plunger which is slidably mounted within the channel. The plunger is adapted for movement withinthe wide section and narrow section of the flow channel wherein the clearance distance between the sidewall of the narrow section and the plunger surface extending into the narrow section is substantially constant. When the plunger is moved reciprocally into the narrow section the changes in pressure'drop across the valve or apparatus are substantially directly proportional to the axial extent to which the plunger lies within the narrow section.

11 Claims, 4 Drawing Figures PATENTEDMAR 7 I972 SHEET 1 OF 4 IN V E NTORfi 0. s K I so ANDREW CHESTER L. WOODWORTl-l (Animal/n01 PATENTEDMAR 71972 ,5 7, 4

SHEET 2 [1F 4 i w If N'TORS ANDREW D. SKIEO CHESTER L. WOODWORTH PATENTEDMAR 7 I972 SHEET 3 [IF 4 VALVE PREfiSURE DROP VERSUS LAND LENGTH STEM POSITION 0.010 wNtJmwwml w J 1 a H F 0 E M v E M. v D R A 0 N m 1 /l\. l o o 0 o o 0 o o o w o w o o w o o o 6 4 0 6 6 4 0 0 0 0 O 2 a a 2 1 I E I 8 6 4 2 LAND LENGTH OF VALVE OP FIG. 1

v (INCHES) mum POSITION OF STANDARD VALVE O F'LJLL (JPEIN IN ENTORS ANDREW 0. 5K1 150 LHETJTER L WOOIDWORTH BY wwum Plmm rm Cuttoflmu,

PATENTVEDMAR 7 m2 SHEET H []F 4 INVENTORS ANDREW D. SKIBO CHESTER L .WOODWORTH BY -LMLW& FAA/Wm alike-R1457 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a pressure control valve or apparatus for controlling the pressure of a viscous fluid in a conduit, and more specifically, for producing changes in pressure which are directly proportional to the valve stem position.

2. Description of the Prior Art It is known to the art that valves can be used to control the pressure of a fluid in a conduit by varying the cross-sectional area of the conduit. However, most of the valves currently available do not provide a means for accurately controlling the pressure over wide ranges. The pressure drop across these valves is related exponentially to the position of the valve stem. As a result it becomes extremely difficult to obtain small changes in pressure when the valve stem is near the closed position and large pressure drops are occuring across the valve. These valves usually incorporate channels which are not fully streamlined and valve seats of conical or other type configurations which are designed to completely shut off the flow. There have been attempts to design pressure control valves in which the pressure is directly proportional to the valve stem position. However, these designs are frequently based on complicated and expensive mechanical linkages that vary the ratio of stem movement to plunger movement. There have also been attempts to achieve more precise pressure control by including flow channels of varying dimensions. However, these valves have produced extremely poor response curves relating pressure drop to valve stem position. Furthermore, the flow stagnation Zones found in many of these concepts becomes a significant problem when a thermally degradable material is being controlled as in an extrusion line.

SUMMARY OF THE INVENTION Now there has been developed a new combination pressure control valve, hereto unknown to the prior art, which resolves the problems described above. In particular, the valve has been designed to produce a response curve in which pressure drop is substantially linearly related to valve stem position over its entire range of operation. The valve is also designed to eliminate any dead spots by having a completely streamlined flow channel which prevents material holdup and the subsequent problems of degraded fluid inclusions.

Accordingly, the main object of the present invention is to provide an improved pressure control valve for controlling the pressure of a viscous fluid in a conduit.

Another object of the present invention is to provide a pressure control valve which produces substantially linear pressure drop changes in response to changes in the valve stem position during flow of viscous fluid through the valve.

An additional object of the present invention is to provide an improved pressure control valve for controlling the back pressure at the outlet end of an extruder for molten thermoplastic material.

A further object of the present invention is to provide a pressure control valve having a streamlined flow channel which minimizes material stagnation in the channelt A still further object of the present invention is to provide a leakproof pressure control valve for a viscous fluid.

Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter.

These and other objects are attained by providing a pressure end of the valve stem and coextensive therewith, the plunger having a first portion of substantially uniform cross-sectional dimension and a streamlined end portion, the plunger being adapted for movement within the wide section and the narrow section, wherein the clearance distance between the sidewall of the narrow section and the plunger surface extending into the narrow section is substantially constant, and wherein the cross-sectional clearance area between the wide section and the plunger surface lying within the wide section is at least two times as large as the cross-sectional clearance area between the narrow section and the plunger surface extending into the narrow section; and means for reciprocally moving the valve stem and the plunger along their longitudinal axis; wherein the pressure drop change across the narrow section during flow of the viscous fluid through the valve is substantially directly proportional to the axial extent to which the plunger lies within the narrow section.

BRIEF DESCRIPTION OF THE DRAWINGS In describing the overall invention, reference will be made to the accompanying drawings in which:

FIG. 1 is a side view section of a preferred embodiment of a pressure control valve for controlling the pressure of a viscous fluid in a conduit;

FIG. 2 is a side view section of an alternate embodiment of the invention; and

FIG. 3 is a graph illustrating the relationship between the pressure drop and plunger position of the valve shown in FIG.

control valve adapted for controlling the pressure of a viscous fluid in a conduit, the valve comprising in combination a valve body; a streamlined flow channel within the valve body, the flow channel having a narrow section of substantially uniform cross-sectional dimension which opens into a wide section of larger cross-sectional dimension than the narrow section; a valve stem which is slidably mounted for reciprocal axial movement within a stem guide and within the flow channel; an axially disposed plunger which is connected to the terminal FIG. 4 is a side view section of an embodiment of the invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS With reference to the drawings, there is shown in FIG. 1 a pressure control valve 10, including a valve body I2,'a streamlined annular flow channel 14, a valve stem 16, and a plunger 18.

The valve body 12 has a generally cylindrical configuration and houses a stem guide 20 and the flow channel 14.

The streamlined annular flow channel 14 has a narrow section 22 of substantially uniform cross-sectional dimension. The narrow section 22 opens into an outwardly flaring wide section 24 of larger cross-sectional dimension than the narrow section 22. Section 24 flares initially outwardly from the axis of the narrow section in a streamlined configuration and curves to form a substantially right angle with the axis of the narrow section.

The valve stem 16 is slidably mounted for reciprocal axial movement within the stem guide 20 and within a portion of the wide section 24 and optionally within the narrow section 22 of the flow channel. The stem 16 has a small cross-sectional area in relation to the wide section 24 in order to permit streamlined flow around all sides of the stem and minimize stagnation on the side of the stem opposite to the direction of flow.

The axially disposed plunger 18 is connected to the lower terminal end of the valve stem 16 and coextensive therewith. The 'plunger 18 has a first portion 28 of uniform cylindrical configuration and a streamlined end portion 30. The longitudinal axis of the plunger 18 is coaxial with the longitudinal axis of the valve stem 16. The plunger is adapted for reciprocal movement within the wide section 24 and narrow section 22 of the flow channel 14. The clearance distance between the sidewall 32 of the narrow section and the surface 34 of the cylindrical portion 28 ofthe plunger 18 extending into the narrow section 22 is substantially constant along its axial extent.

The dotted line in the narrow section 22 of the flow channel 14 illustrates the position of the plunger 18 when it is partially extending into the narrow section 22. The land length 36 is a variable dimension and refers to the axial extent to which the cylindrical first portion 28 of the plunger 18 lies within the narrow section 22. As the cylindrical first portion 28 moves further into the narrow section 22 the land length 36 will increase and the pressure drop across the narrow section 22 will increase proportionately.

The cross-sectional clearance area between the surface 34 of the first portion 28 of the plunger 18 extending into the wide section 24 of the channel 14 and the sidewall 33 of the wide section 24 must be at least two times as large as the crosssectional clearance area between the sidewall 32 of the narrow section 22 and the plunger surface 34 extending into the narrow section 22. This difference in cross-sectional clearance area is necessary since substantially no pressure drop should occur across the wide section 24 of the channel regardless of changes in the land length 36 due to the movement of the plunger 18. However, the pressure drop across the narrow section 22 is substantially directly proportional to the land length 36.

When the viscous fluid flowing through the valve is molten thermoplastic, the clearance distance in the'narrow section 22 is preferably between 0.003 and 2.0 inches, the land length 36 is preferably between 0.25 and 15.0 inches, and the diameter of the plunger is preferably between 0.1 and 6.0 inches. The operating range of the valve will depend on the two interrelated parameters, clearance distance and land length. For example, if large pressure drops are to be achieved for a small land length 36, the clearance distance between the plunger surface 34 and the sidewall 32 of the narrow section 22 must be decreased. In contrast, a channel having a larger clearance distance in the narrow section 22 will decrease the pressure drop for a particular land length 36.

A means for reciprocally moving the valve stem 16 and the coextensive plunger 18 along their longitudinal axis is provided such as an automated hydraulic or air cylinder. The valve stem 16 and the plunger 18 can also be positioned manually with a conventional rotatable handle, if desired.

Also provided is a valve-packing gland 40 to hold a packing material 42 which sealingly surrounds the valve stem 16 and prevents leakage of the viscous material. The lower portion of the valve packing gland 40 and packing material 42 lie within stem guide and have respective clearances which permit reciprocal movement of the valve stem 16 through the gland 40 and packing material 42.

When the plunger 18 extends into the narrow section 22 of the channel 14 excessive heat may be generated by the passage of viscous fluids through the narrow section 22. This temperature variation in the fluid will cause a variation in the flow characteristics which would detract from the linear flow control of the valve. This would be due to lower pressure drops attained in the narrow section 22. Furthermore, a

distortion of the temperature profile may be produced by the concentric tubes extending parallel to the axis of the plunger l8 and stem 16. A temperature controlled fluid is passed through the tubes to cool the surface of the plunger 18. It is preferable to have the cooling fluid pass down into the plunger through an inner tube and then up the plunger towards an outlet through an outer annulus surrounding the inner tube. In this way heat transfer to the cooling fluid will be minimized until the fluid reaches the lower portion of the plunger 18, where the generation of heat is greatest.

A second temperature control means may also be optionally provided to dissipate any heat generation within the narrow section 22 or alternately to provide additional heat during the initial passage of fluid through the valve. This second temperature control means may consist of a jacket surrounding the wall 32 defining the narrow section 22. The jacket will have an inlet port and an outlet port through which a temperature controlled fluid may be passed. A cooling fluid will be used to reduce the effects of shear heat within the narrow section 22 during operation of the valve.

A warm fluid will be used to preheat the narrow section prior to startup. Other conventional cooling and heating devices such as blowers may be used to control the temperature of the wall defining the narrow section 22.

FIG. 2 illustrates a particularly attractive embodiment of a pressure control valve 44 in which the flow channel 46 has a more streamlined configuration than that illustrated in FIG. 1. The flow channel 46 has a flattened S configuration to prevent material buildup and provide a reduced Pressure drop across the valve when the entire plunger 48 is lifted into the wide section 50 of the flow channel 46. The plunger 48 and stem 52 can be of substantially equal and uniform dimension along their axial lengths and thus form a uniform integral unit. This uniform configuration for the plunger 48 and valve stem 52 has been found to be more effective in controlling back pressure at the outlet end of an extruder when molten thermoplastic is being extruded at lower flow rates and lower line pressures. The dotted line in the narrow section 54 of the flow channel 46 illustrates the plunger 48 in the maximum land length 56 position to produce a maximum pressure drop across the valve 44.

FIG. 4 illustrates the use of heating and cooling means in the plunger 48 and in the narrow section 22 of the flow channel 46. An inlet conduit 65 and an outlet conduit 66 is provided in the stem 52 and plunger 48 which carries heating or cooling fluids for controlling the temperature of the plunger. FIG. 4 also illustrates the use of a jacketed coil 61 in the wall of the narrow section of the flow channel 22 for controlling the temperature of the narrow section. Heating or cooling fluids are introduced at the coil inlet 62 circulated through the coil around the narrow'section to the coil outlet 63. Also illustrated are heaters 64 with lead in wires 67 on the outside wall of the narrow section 22 which may be used to control the temperature of the narrow section of the flow channel.

The graph of FIG. 3 illustrates the relationship between valve pressure drop and land length for the valve shown in FIG. 1, as compared with the relationship between pressure drop and stem position for a standard pressure control valve when molten thermoplastic material passes through each valve which is located at the outlet of a conventional extruder for molten thermoplastic resins. The graph indicates the substantially linear relationship between the pressure drop and land length 36 for various positions of theplunger 18 of FIG. 1. Because the pressure drop is directly proportional to the land length 36 over the entire operating range of the valve of FIG. 1 small changes in pressure can be precisely controlled. The only limitation on the operating range of the valve of FIG. 1 is the maximum land length 36 and the cross-sectional clearance distance in the narrow section 22 of the channel 14.

In contrast, the pressure drop produced by the standard pressure control valve changes exponentially with changes in the stem position. The graph of the standard valve shows the dramatic increase in pressure drop when the stem position is greater than 0.3 inch. It is apparent that small changes in pressure drop are extremely difficult to control with the standard valve when the pressure drop is greater than 400 psi.

The pressure drop of the materials passing through the conduit is substantially linear. However, some slight variation from this linear relationship may occur if the material passing through the valve undergoes a temperature change. Of course, this may be controlled in some cases by maintaining constant temperature conditions.

It should be apparent from FIG. 3 and the foregoing description that an important contribution is made by the present invention. This invention allows the precise control of pressure of a viscous fluid passing through a conduit. As stated above such precise control cannot be achieved by the valves of the prior art.

In general, the preferred materials used in the construction of the valves of the present invention are soft and hardened steels, aluminum, and other metals which are easily machined. Low-carbon 10-20 steel is especially suitable for the valve body and the packing gland. The valve stem and plunger can be machined as one integral unit and a wear-resistant material such as graph-mo steel, oil hardened to R, 60-64, has been found to be effective. The stem guide can be made of a slightly softer material than the stem and plunger, such as graph-mo steel, oil hardened to R 45-50. A preferred material for the packing is graphite asbestos.

The present invention finds utility in any system in which the pressure of a viscous fluid in a conduit must be precisely controlled. Molten synthetic resin such as polyvinyl chloride, polystyrene, polyvinyl butyral, polyethylene, and other thermoplastic materials, and petroleum products such as crude oil, refined oil, domestic fuel, and automotive fuel are examples of the viscous fluids which may be controlled with this valve. Other fluids which can be controlled include ketones, esters, alcohols, etc. The pressure drop produced by the valve is substantially directly proportional to the land length because of the relationship between the configuration of the channel and the plunger. The substantially linear changes in pressure drop are achieved regardless of the composition of the viscous fluid being controlled. The flow channel has a completely streamlined configuration to prevent material holdup. This feature is particularly important when the valve is used to control back pressure at the outlet end of an extruder for molten thermoplastic material which is subject to thermal degradation.

Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that many variations and modifications of the details of construction and combination and arrangement of parts herein described will be obvious to those skilled in the art and may be carried out without departing from the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. An apparatus adapted for controlling the back pressure at the outlet end of an extruder for molten synthetic resin material, said apparatus comprising in combination:

a. a valve body;

b. a streamlined flow channel within said valve body, said flow channel having a streamlined narrow section of substantially uniform cross-sectional dimension which opens into a streamlined wide section of larger cross-sectional dimension than said narrow section;

c. a valve stem which is slidably mounted for reciprocal axial movement within a stem guide and within said flow channel;

, d. an axially disposed plunger which is connected to the terminal end of said valve stem and coextensive therewith, said plunger having a first portion of substantially uniform cross-sectional dimension slightly smaller than said narrow section and a streamlined end portion, said plunger being adapted for movement within said wide section and said narrow section, wherein the clearance distance between the sidewall of said narrow section and the surface of said first portion of said plunger extending into said narrow section is substantially constant, and wherein the cross-sectional clearance area between said wide section and the surface of said first portion of said plunger lying within said wide section is at least two times as large as the cross-sectional clearance area between said narrow section and said surface of said first portion of said plunger extending into said narrow section; and

e. means connected to said stem for reciprocally moving said valve stem and said plunger along their longitudinal axis;

wherein the pressure drop change across said narrow section during flow of said molten synthetic resin material through the flow channel, is substantially directly proportional to the axial extent to which the end portion of said plunger which lies within said narrow section.

2. The apparatus of claim 1 wherein said first portion of said plunger has a generally cylindrical configuration and said narrow section of said channel has a generally cylindrical configuration of slightly larger diameter than said plunger.

3. The apparatus of claim 1 wherein said streamlined end portion has a generally conical configuration.

4. The apparatus of claim 1 wherein said wide section has a gradually flaring streamlined configuration, said wide and narrow sections being adapted for the streamlined flow of molten resin within said flow channel.

5. The apparatus of claim 1 wherein the diameter of said plunger is between 0.1 and 6.0 inches and the diameter of said narrow section of said channel is between 0.102 and 8.0 inches.

6. The apparatus of claim 1 wherein the diameter of said plunger and the diameter of said stem are substantially equal and uniform along their longitudinal axis.

7. The apparatus of claim 1 wherein the plunger has a temperature control means to control the temperature of the plunger.

8. The apparatus of claim 1 wherein a temperature control means is associated with the wall defining said narrow section of the flow channel.

9. The apparatus of claim 1 wherein the temperature control means comprises a channel in said plunger, said channel being adapted for the passage of temperature controlled fluid through said plunger.

10. The apparatus of claim 1 wherein the temperature control means includes a jacket surrounding the wall defining said narrow section of said flow channel, said jacket having an inlet port and an outlet port for the passage of temperature controlled fluid.

11. An apparatus adapted for controlling the back pressure at the outlet end of an extruder for molten synthetic resin material, said valve comprising in combination:

a. a valve body;

b. a streamlined flow channel within said valve body, said flow channel having a streamlined narrow section of substantially cylindrical configuration which opens into a gradually flaring streamlined wide section of larger crosssectional dimension than said narrow section, a portion of said wide section curving to form an angle with the axis of said narrow section, said angle being between 10 and c. a valve stem which is slidably mounted for reciprocal axial movement within a stem guide and within said flow channel;

d. an axially disposed plunger which is connected to the terminal end of said valve stem and coextensive therewith, said plunger having a first portion of substantially cylindrical configuration and a streamlined end portion, said plunger being adapted for reciprocal movement within said wide section and said narrow section, the maximum axial extent to which said first portion of said plunger may move into said narrow section being between 0.25 and 15.0 inches, wherein the clearance distance between the sidewall of said narrow section and the surface of said first portion of said plunger extending into said narrow section is substantially constant along the axial extent of said first portion of said plunger, said clearance distance being between about 0.003 and 2.0 inches, and wherein the cross-sectional clearance area between said wide section and the surface of said first portion of said plunger lying within said wide section is at least two times as large as the cross-sectional clearance area between said narrow section and said surface of said first portion of said plunger extending into said narrow section;

e. means connected to said stem for reciprocally moving said valve stem and said plunger along their longitudinal axis;

f. means within the plunger for controlling the temperature of the plunger; and

g. means for controlling the temperature of the wall defining said narrow section,

wherein the pressure drop change across said narrow section during flow of said molten synthetic resin material through the flow channel is substantially directly proportional to the axial extent to which the end portion of said plunger lies within said narrow section. 

1. An apparatus adapted for controlling the back pressure at the outlet end of an extruder for molten synthetic resin material, said apparatus comprising in combination: a. a valve body; b. a streamlined flow channel within said valve body, said flow channel having a streamlined narrow section of substantially uniform cross-sectional dimension which opens into a streamlined wide section of larger cross-sectional dimension than said narrow section; c. a valve stem which is slidably mounted for reciprocal axial movement within a stem guide and within said flow channel; d. an axially disposed plunger which is connected to the terminal end of said valve stem and coextensive therewith, said plunger having a first portion of substantially uniform crosssectional dimension slightly smaller than said narrow section and a streamlined end portion, said plunger being adapted for movement within said wide section and said narrow section, wherein the clearance distance between the sidewall of said narrow section and the surface of said first portion of said plunger extending into said narrow section is substantially constant, and wherein the cross-sectional clearance area between said wide section and the surface of said first portion of said plunger lying within said wide section is at least two times as large as the cross-sectional clearance area between said narrow section and said surface of said first portion of said plunger extending into said narrow section; and e. means connected to said stem for reciprocally moving said valve stem and said plunger along their longitudinal axis; wherein the pressure drop change across said narrow section during flow of said molten synthetic resin material through the flow channel, is substantially directly proportional to the axial extent to which the end portion of said plunger which lies within said narrow section.
 2. The apparatus of claim 1 wherein said first portion of said plunger has a generally cylindrical configuration and said narrow section of said channel has a generally cylindrical configuration of slightly larger diameter than said plunger.
 3. The apparatus of claim 1 wherein said streamlined end portion has a generally conical configuration.
 4. The apparatus of claim 1 wherein said wide section has a gradually flaring streamlined configuration, said wide and narrow sections being adapted for the streamlined flow of molten resin within said flow channel.
 5. The apparatus of claim 1 wherein the diameter of said plunger is between 0.1 and 6.0 inches and the diameter of said narrow section of said channel is between 0.102 and 8.0 inches.
 6. The apparatus of claim 1 wherein the diameter of said plunger and the diameter of said stem are substantially equal and uniform along their longitudinal axis.
 7. The apparatus of claim 1 wherein the plunger has a temperature control means to control the temperature of the plunger.
 8. The apparatus of claim 1 wherein a temperature control means is associated with the wall defining said narrow section of the flow channel.
 9. The apparatus of claim 1 wherein the temperature control means comprises a channel in said plunger, said channel being adapted for the passage of temperature controlled fluid through said plunger.
 10. The apparatus of claim 1 wherein the temperature control means includes a jacket surrounding the wall defining said narrow section of said flow channel, said jacket having an inlet port and an outlet port for the passage of temperature controlled fluid.
 11. An apparatus adapted for controlling the back pressure at the outlet end of an extruder for molten synthetic resin material, said valve comprising in combination: a. a valve body; b. a streamlined flow channel within said valve body, said flow channel having a streamlined narrow section of substantially cylindrical configuration which opens into a gradually flaring streamlined wide section of larger cross-sectional dimension than said narrow section, a portion of said wide section curving to form an angle with the axis of said narrow section, said angle being between 10* and 140*; c. a valve stem which is slidably mounted for reciprocal axial movement within a stem guide and within said flow channel; d. an axially disposed plunger which is connected to the terminal end of said valve stem and coextensive therewith, said plunger having a first portion of substantially cylindrical configuration and a streamlined end portion, said plunger being adapted for reciprocal movement within said wide section and said narrow section, the maximum axial extent to which said first portion of said plunger may move into said narrow section being between 0.25 and 15.0 inches, wherein the clearance distance between the sidewall of said narrow section and the surface of said first portion of said plunger extending into said narrow section is substantially constant along the axial extent of said first portion of said plunger, said clearance distance being between about 0.003 and 2.0 inches, and wherein the cross-sectional clearance area between said wide section and the surface of said first portion of said plunger lying within said wide section is at least two times as large as the cross-sectional clearance area between said narrow section and said surface of said first portion of said plunger extending into said narrow section; e. means connected to said stem for reciprocally moving said valve stem and said plunger along their longitudinal axis; f. means within the plunger for controlling the temperature of the plunger; and g. means for controlling the temperature of the wall defining said narrow section, wherein the pressure drop change across said narrow section during flow of said molten synthetic resin material through the flow channel is substantially directly proportional to the axial extent to which the end portion of said plunger lies within said narrow section. 